Wafers are used in the Semiconductor and Solar industry. Typically they are round or have a stable, rectangular shape. Semiconductor wafers of 4″ up to 12″ are used with a standard thickness between 500 μm up to 875 μm. But there active elements, e.g. transistors have a pn-junctions depth of only 0, 1 to 5 μm. Thus, most of the material of the pure, often defect free wafers is used as an expensive carrier material to enable a reliable, stable mechanical handling and processing of the wafers within different production environments. Once those wafers become thin (after grinding they are less than 150 μm) they start to bow because of the gravity or other stress generating factors like grinding imperfections, crystal defects, thermal stress etc. For example a stress relieved (100) oriented 8″ Silicon wafer of 150 μm thickness, placed in a standard vertical wafer boat, shows a bow of about 2 mm from gravity stress only. Even a different bow behaviour in [110] and [100] directions occurs, because of there mechanical anisotropy of Young's Modulus (E) of about 1.7*E+11 Pa in [110] and 1.3*E+11 Pa in [100] directions. This variation can also course saddle like warpage. By the way, (111) wafers do not show this anisotropy in directions perpendicular to [111], the Young's Modulus value in those directions is constant at 1.7*E+11 Pa. With decreasing wafer thickness, warpage and bow increase significantly and can get unpredictable, so automated wafer handling becomes more and more challenging. Wafers with less than 100 μm even become foil-like with a transition from rigid to flexible, but they are also becoming even more brittle. So there is a need to protect those thin wafers against breakage and, if possible, keep them flat all time. The preferred solution is to clamp those thin wafers permanently on a mobile, electrostatic chuck.
Stationary electrostatic chucks are used to clamp wafers or other disk-shaped conducting or semi-conducting substrates, in particular as holding device in semiconductor processing equipments, for many years. The operating principle is described in detail in publications such as: Watanabe et. al.: Jpn. J. Appl. Phys., Vol. (32), 1993 864-871 and Mahmood Naim: Semi-conductor Manufacturing, August 2003, 94-106. Stationary electrostatic chucks are installed within many process tools and they are permanently connected to a power supply unit. Because those chucks have a fixed place within the tool, they can not be used in a mobile mode. By employing different materials for the dielectric insulation of the chucks, different leakage currents occur.
Chucks, which take advantage of the Johnsen-Rahbeck-effect, use a generated small current flow to increase the clamping force. The needed current is supplied from a permanently connected power unit. Technical solutions are described in U.S. Pat. No. 6,174,583, U.S. Pat. No. 5,151,845, U.S. Pat. No. 5,909,355, U.S. Pat. No. 6,268,994 as well as EP 0768389. Those ceramic chucks, made by thick-layer-technologies, show a high voltage breakdown resistance, are able to withstand high temperatures and have good resistance against different chemical and plasma processes. Chucks, which allow only a very small leakage current based on the material selection, are called Coulomb-Chucks. Those chucks have a typical specific electrical resistance of the dielectric layer of more than 10E15 Ohm*cm. Especially the use of polyimide foils or PTFE can reach this demand, but also chucks made of high resistant Al2O3-ceramics or silicon carbide are in use (see also U.S. Pat. No. 5,255,153, EP 0 693 771, EP 0 948 042 and U.S. Pat. No. 6,483,690). Those chucks can work in a unipolar mode (mostly in conjunction with a plasma process) as well as in bipolar modes.
The methods for applying these principles to mobile, transportable electrostatic support carriers are described in detail in EP 1217655A1, US 2002/0110449A1, FR 2774807, U.S. Pat. No. 4,551,192 and WO/02 11184 A1, they represent the prior art. Mobile, transportable electrostatic chucks, so-called “Mobile Chucks”, are used for mechanical holding of film-like materials. Those support carriers allow a safe manipulating of thin, brittle wafers on existing production equipment and the storage in or between production steps because the size and thickness of the clamped substrate on the mobile, transportable electrostatic chuck is similar of size, thickness and shape as a standard wafer. The practical application of this new manipulation method resulted in the development of first samples of mobile, transportable electrostatic chucks, in particular for the semiconductor industry; compare DE 20311625, DE 102004045447, DE 202005004589 and DE 102004041049. However, the first proposed Mobile Chucks made of ceramic or polyimide materials fulfilled only unsatisfactorily some of the technical and economic requirements as a high particle generation on the surface and a high complexity (cost) during there manufacturing process. All through the risk of breakage of a thin (<150 μm) and ultra thin (<50 μm) substrate is drastically reduced during many manipulation and process steps, there is still a problem with the clamping power in high humidity environments or during wet processes. Typically back side contacts for charging the Mobile Chucks are employed. Those back side contacts can be the root course for an unintentional discharging effect which reduces the clamping force and thus the clamped substrate can get lost. Processes which can be impacted seriously are: grinding, photolithography, wet etch and cleaning.
The most important feature of Mobile Chucks is that they are mobile. That means that they do not need a permanent power supply and for this reason they can be moved or transported inside or outside of process tools without limitations. Mobile Chucks are Coulomb-Chucks. The goal is to eliminate leakage currents or reduce them drastically. This means that the insulation effect of the dielectric layer must be very good in order to achieve a long lasting clamping effect. After charging, the energy is stored in a capacitor structure which is build up between the Mobile Chuck electrode/dielectric layer/wafer (substrate to be clamped). High leakage currents would quickly reduce the stored energy and this would only allow a short clamping time. In addition, a good insulation of the electrodes against humidity is desirable because unintentional discharging of the electrodes can happen due to the open contact area, which is accessible to the outside environment. Thus the risk is high that the clamping force between the thin wafer and the Mobile Chuck is reduced to early. The clamping force of Coulomb Chucks is proportional to the square of the applied clamping voltage (U), the dielectric constant (∈r) of the insulator layer and is indirectly proportional to the square of the thickness of the insulator layer (d). Thus, a strong electrostatic holding force is obtained by having a high clamping voltage (U ˜1000 V) with a high dielectric constant of the material (∈r>3.5) and a very small thickness of the insulation layer (d). Obviously, high clamping forces for Mobile Chucks can be achieved using thin film technologies.
The insulation of open contact areas of Mobile Chucks, which is accessible to the outside environment, has been discussed in some patents mentioned above. They propose special insulation layers, which are building up on top of the open contact area in order to insulate the electrode from the surrounding environment, process conditions like humidity or liquids. In particular protection layers from so called ductile insulation materials made of silicone or polyethylene-foils, on top deposited layers of electrically insulating semiconductor materials or mechanical- or electromagnetic switches are proposed to avoid unintentional discharging effects. Protection layers made of silicone or polyethylene-foils are typically 10 μm to 100 μm thick. This is a clear disadvantage because those kinds of layers have strong limitations during the semiconductor IC manufacturing process. Mechanical- or electromagnetic switches can be produced using methods known from micro-system-technology, but there integration as well as the additional deposition of semiconductor layers will generate high manufacturing costs. The use of Schottky diodes on top of the open contact areas are also not sufficient, because there breakdown voltage is relatively low and there leakage current is relatively high in comparison with pn-junction diodes.